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Solving a mysterious misfire on an MGBGTV8

Barrie Franklin (Midnight Blue 1538) from Bedfordshire posted a note on the V8BB seeking help with a misfire on his V8. Here he explains how it was solved. (May 05)

Last December I posted a note on the V8BB asking for members' ideas to help trace the cause of an intermittent misfire on my V8. I explained that my factory MGBGTV8 was starting easily and ran smoothly for half an hour, but then the engine seemed to misfire, particularly on a small throttle opening at lowish speeds in 3rd or 4th gear. A similar problem had been cured two years earlier when the distributor was professionally rebuilt. Sadly the fault had started again. I mentioned that I had replaced the plugs, HT leads, distributor cap, rotor arm, fuel filter and coil. The electrical connections to the Luminition unit had been cleaned. Even removing the petrol filler cap did not release a vacuum. The carburettors had been cleaned, the settings checked and a pressure test done on the engine when cold and hot. A friendly local garage put it on a Crypton tester which did not show any faults. I have always run on branded petrol with ValveMaster Plus and recently changing to Shell Optimax and ValveMaster Plus has not helped. I ended the posting with a plea "Has anyone any suggestions?"

Victor Smith responded commenting that "one cause of misfiring can be traced to battery terminals. The old top hat connectors are dreadful and can cause misfiring on pick-up from low revs. Replacing the connectors with the clamp type is a very good improvement and it also gives a better clearance between the top of the connector and the underside of the metal panel over the battery box".

Bob Owen commented queried whether the misfire was "temperature dependent? Are you using K&N filters? The temperature of the air in the underbonnet gets high if stationary or travelling slowly and K&Ns draw from near the top causing the mixture to go out. Mine was cured by setting just weak of peak tickover rather than on the rich side; note that the carburettors are always set and checked with the bonnet open!"

Chris Armitage responded "I had a similar problem on my factory V8 a few years ago. In the end I disconnected the Luminition optical ignition system and reverted to the original contact breakers. Problem solved. Incidentally one of the most effective

improvements I have made to the car is to replace the Lucas distributor with a Mallory twin point item. In my experience this is a much better engineered device". Chris subsequently wrote up this modification as a V8 Workshop Note.

Dave Wellings added "heat sensitive misfiring is often related to something on the ignition side breaking down. If you have not checked the manual regarding the HT leads, note also that there is a prescribed layout and order for the HT leads which avoids two specific leads being adjacent to each other. Where is your Lumenition mounted? Ideally it should be ahead of the radiator panel, in front of the coil which is a much cooler place than the engine side. If all else fails try it on points. At least that will eliminate the electronic ignition as the cause of the misfire problem. One final thing - is the misfire linked to the fans coming on? I suffered an infuriating fault on a Lumenition unit fitted to an MGB 1800 which was caused by voltage fluctuation when the fans kicked in".

I checked the car using Dave's suggestions and posted a note saying I had followed his suggestion to see if the misfire occurred once the cooling fans had come on but found there was no link so was no nearer an answer. The car runs well until hot enough to bring on the fans and then it misfires under light load. Disconnecting the wire from the temperature sensor to stop the fans did not stop the misfire. I then asked Dave if he had any other ideas other than reverting to points to eliminate the Luminition. I followed up most of the useful suggestions from the other members but none solved the problem.

After having the carburettors rebuilt by Carburettor Exchange of Leighton Buzzard, I eventually booked the car in for a rolling road investigation at Wilshire Garage at Wimpole near Cambridge. The problem was soon identified - the petrol pump was supplying fuel at 5psi measured between the filter and the carburettors whereas Wilshire explained it ought to be 2.0 to 2.5psi. A pressure adjusting valve was fitted, set at 2psi and the timing and carburettors adjusted slightly. Result - problem solved! In fact the car is now running far better than any time in my nine years of ownership. It was also a very interesting experience "driving" on a rolling road - doing 100mph on the clock and going nowhere!

Thanks again to all who contributed their ideas via the V8BB.